New wireless technology is being developed and deployed to provide support for voice and multimedia services in both residential and enterprise environments. Wireless Local Area Network (“WLAN”) devices, for example, are being developed in conjunction with IEEE 802.11 standards to support packetized voice communications such as Voice over Internet Protocol (“VoIP”). There are technological hurdles that must be overcome in order to support voice and multimedia on WLANs because the technology was initially designed to support simple data communications. In particular, voice and multimedia applications can be more sensitive to jitter, delay and packet loss than data communications applications. IEEE 802.11 is under development, and continually provides new protocols and techniques which seek to overcome some of these technological hurdles as well as to increase the capacity of a wireless network.
Because the costs associated with developing, purchasing, selling and deploying a new wireless technology are often quite high, it is common to conduct testing to mitigate the risk that the technology will fail to perform as planned. However, wireless devices are notoriously difficult to test because they can be affected by ambient sources of interference. Further, the conditions to which a wireless device may be subjected in actual use are so great in number that it is difficult and time-consuming to create all of those conditions in a test environment. It is known, for example, to simulate some wireless network operations by manually moving a wireless device through a building in which wireless access devices are strategically situated. However, this technique is too labor-intensive and imprecise to simulate a wide variety of traffic conditions, distances between access points and rates of motion in a practical manner. Further, such a manual, open-air test can be rendered invalid by transient interference from a microwave, RADAR or other RF source. More recently it has become known to simulate a wireless network by enclosing devices in EMI-shielded containers which are in communication via wired connections. Such a system is disclosed in U.S. Pat. No. 6,724,730 entitled “Test System for Simulating a Wireless Environment and Method of Using Same”, by Mlinarsky et al. (herein after the Mlinarsky patent) which is incorporated herein by reference.
FIG. 1 illustrates the prior art architecture of Mlinarsky. A central RF combiner 110 connected to a plurality of connection nodes 102 via programmable attenuation components 108. A controller console controls the programmable attenuation component for the purposes of simulating spatial positioning of the plurality of connection nodes to facilitate operational testing of the nodes. As shown in FIG. 1, the RF combiner arrangement enables simulation of movement by the coupled nodes along the links of the star topology. While this architecture is effective for simulating movement within the topology, the simulation of multi-dimensional movement is restricted by the available connections. It would therefore be desirable to identify an improved architecture which is capable of providing full nodal connectivity to simulate movement in multiple dimensions.
In addition to identifying an architecture with increased movement simulation capabilities, it would also desirable to identify a wireless test architecture capable of adequately testing the operation of Multiple Input, Multiple Output (MIMO) devices as defined in IEEE 802.11n™. 802.11n is new standard for high-speed wireless local area networking, offering throughput greater than 100 Mbps. 802.11n works by utilizing multiple wireless antennas in tandem to transmit and receive data. The associated term “MIMO” refers to the ability of 802.11n (and other similar technologies) to coordinate multiple simultaneous radio signals. MIMO increases both the range and throughput of a wireless network by taking advantage of the distinguishability of signals transmitted on the same FCC allocated radio channel by different radios.
In general MIMO uses multiple antennas to send multiple distinct signals across different spatial paths at the same time, increasing throughput. The radio signals are naturally reflected, absorbed and diffracted as they propagate through different materials in any enclosed space. The reflections arrive at a receiver with unpredictable amplitude, time and phase relationships, causing multipath distortion of the original signal. High data-rate signals are more susceptible to multipath, which has traditionally limited speed and range. The higher the data rate, the more detrimental the multipath distortion is to the signal. MIMO signal processing exploits-the fact that each different spatial path has different multipath, by essentially ‘training’ the receivers to associate the differently distorted received signals with different radios. This allows MIMO receivers to recover the multiple distinct transmitted signals.
A variety of wireless products will shortly be introduced that operate according to the 802.11n protocol. Prior to their introduction, it will be desirable for vendors to identify methods of testing their devices in order that they may verify the products' ability to operate according to the protocol, and also to quantify the capabilities of their product. It would therefore be desirable to identify a test architecture which would permit verification of devices operating under the 802.11n protocol.